We are investigating the mechanisms of action and regulation of the immunoglobulin heavy chain constant region gene switch recombinase as a function of B cell physiology and development. Switch substrate retroviruses (SSRs) will be introduced into cultures of mature murine and human B lymphocytes, purified subsets of human B cells and murine pre-B cells to assess the activation, regulation and developmental timing of switch-recombinase activity. Model B cell lines responsive to CD40 and cytokine signaling will be employed to discriminate between extracellular signals that initiate and/or potentiate switch-recombinase activity. Sensitive PCR assays will be performed to reveal the consequences for SSR recombination as these cells are exposed to CD40 receptor signaling, cytokines, and dendritic cells known to affect endogenous antibody class switching. SSR harboring CH locus regulatory elements and RNA maturation signals will be employed to determine their putative role(s) in targeting CH gene segments for switch recombination.